Under certain conditions, Boiling Water Reactors (BWRs) are susceptible to coupled neutronic-thermal hydraulic instabilities. Such instabilities, which are characterized by periodic core power and hydraulic oscillations, can compromise established fuel safety limits. Industry events and analytical studies during the past several years have revealed that, in general, existing neutron monitoring features of the reactor protection system do not assure automatic protection against this class of events.
Various organizations within the United States nuclear industry are actively pursuing permanent solutions to this problem that, in accordance with regulations governing the general design criteria for nuclear power plants, will automatically "assure that power oscillations which can result in conditions exceeding specified acceptable fuel design limits are not possible or can be reliably detected and suppressed." Development of these solutions is progressing, and modifications to reactor protection systems may occur within several years. One proposed solution is to provide instrumentation which can detect oscillations after they have commenced, and take appropriate control measures necessary to suppress them. An example of a system for doing so is shown in U.S. Pat. No. 5,174,946. However, implementing such a solution requires substantial modification to existing reactor instrumentation and control systems.
Implementation of effective operational strategies, utilizing existing plant capabilities in the near-term and supplementing the permanent solutions, is critical for mitigation of the reactor instability phenomenon.